NEWBIE HELP! RCD tripping is doing my head in...

[slippyr4]

can it really be a borrowed neutral?

The OP can have one light on, and the second trips the RCD.

If you take the lowest likely power bulb, eg 9W, then that's 300 mA, so should be 10x the current required to trip an RCD if it's using the wrong neutral.

what about a neutral-earth fault with a resistance of a few ohms? that would probably do it. needs an IR test i think.

 

[Chri5]

can it really be a borrowed neutral?

The OP can have one light on, and the second trips the RCD.

If you take the lowest likely power bulb, eg 9W, then that's 300 mA, so should be 10x the current required to trip an RCD if it's using the wrong neutral.

what about a neutral-earth fault with a resistance of a few ohms? that would probably do it. needs an IR test i think.

Yes- Have you ever shorted a dead circuit P to Earth or N to Earth. The RCD will trip on inbalance, hence why the neutral on RCD 1 rail MUST term on the correct neutral bar, same for neutrals on RCD 2.

Such simple fault won't show until the OP tries to use a circuit with such a problem (ie switching the light cct on).

Simple continuity tests would prove if the correct neutral goes to the correct neutral rail.

If the thoughts are towards a borrowed neutral, look at the 2 way switching between grd and 1st landing as the likely problem area.

 

[slippyr4]

i think you missed my point:- the OP can run 1 light per circuit ok: it's when he switches the second light on that the rcd trips.

since a light is probably at least 300mA, it's suspcious - 300mA current differential would trip an RCD. But here it doesn't- it's the additional current from a second lamp on that does.

 

[Goldberg]

can it really be a borrowed neutral?

No, not if the original post:

If all lights are off and then say the kitchen lights go on that trips it as well. Also, I can turn off the kitchen lights, reset the RCD and turn the kitchen on again and it doesn't trip the RCD.

is accurately describing the symptoms.

what about a neutral-earth fault with a resistance of a few ohms? that would probably do it. needs an IR test i think.

I agree - it would, and it does.